The shaped article, for example film, of polyvinyl alcohol is not only excellent in fragrance trapping property, transparency, oil resistance, antistatic property, oxygen barrier property and warmth retention property but is also hydrophilic and water-soluble or dispersible and, therefore, can be used in applications where the usual hydrophobic polymer film is not fully useful.
For the production of film from polyvinyl alcohol, it is common practice to dissolve the polymer in water and cast the solution. While the resulting film is flexible under high-humidity conditions, it has a drawback in that its flexibility is lost in the low-humidity environment. To avoid this disadvantage and insure adequate flexibility under low-humidity conditions, it has been practiced to incorporate a plasticizer such as glycerin in the casting solution.
For the production of film and other shaped articles from polyvinyl alcohol, the method comprising melt-extruding the polymer in a hydrous condition is also known. For assurance of adequate flexibility of the extrudate, a plasticizer is employed in this process, too.
Attempts have also been made to melt-extrude polyvinyl alcohol under substantially anhydrous conditions. However, because the softening point of polyvinyl alcohol is high and close to its thermal decomposition point, it is difficult to melt-extrude polyvinyl alcohol as such. Thus, the melt-molding of polyvinyl alcohol by this technique also involves incorporation of a plasticizer in a substantial amount.
Incidentally, polyvinyl alcohol is not injection-molded for all practical purposes.
While polyvinyl alcohol is produced by polymerizing vinyl acetate and, then, hydrolyzing the resulting polymer, a "copolymerization-modified" polyvinyl alcohol can be obtained by using one or more comonomers in the polymerization stage. Moreover, polyvinyl alcohol can be "post-modified" by permitting a certain other compound to react with the OH group. These modified polyvinyl alcohols have, in addition to the inherent properties of polyvinyl alcohol, some new properties derived from the modifying group introduced.
As such modified polyvinyl alcohols, vinyl alcohol copolymers containing oxyalkylene groups are known. Thus, for example, the following domestic and foreign patent applications describe various modified polyvinyl alcohol polymers.
The specifications of U.S. Pat. Nos. 1,971,662, 2,844,570 and 2,990,398 disclose oxyethylene group-containing polyvinyl alcohols which are obtainable by addition reaction of polyvinyl alcohol to ethylene oxide.
The specifications of U.S. Pat. Nos. 3,033,841 and 4,369,281 disclose oxyalkylene group-containing vinyl alcohol polymers produced by graft-polymerization of vinyl acetate in the presence of a polyalkylene glycol and subsequent hydrolysis.
The specifications of U.S. Pat. Nos. 4,618,648 and 4,675,360 disclose vinyl alcohol-poly(alkyleneoxy) acrylates and claim that these copolymers can be injection-or extrusion-molded and that the resulting shaped articles are water-soluble and flexible.
Japanese Patent Application Kokai No. 59-155408 describes a modified polyvinyl alcohol produced by copolymerizing an oxyalkylene group-containing unsaturated monomer with vinyl acetate and saponifying the resulting copolymer. The specification cites an extensive list of possible applications, among which are shaped articles, but is reticent about relevant production technology.
Japanese Patent Application Kokai No. 1-158016 discloses a water-resistant flexible film from an oxyalkylene group-containing vinyl alcohol polymer. Although the specification mentions, "For the production of film, the method of casting an aqueous solution of this polymer, the method of extruding it in hydrous condition under heating and the melt-extrusion method, among others, can be employed", all the examples in the specification are relevant to the casting method alone and no further description is found of the melt-extrusion method, nor is there a description of the method of extruding the polymer in hydrous condition under heating.
The casting method for polyvinyl alcohol has several disadvantages compared with the melt-extrusion method. Thus, the preparation of an aqueous solution is essential, the film-forming speed is low and the drying procedure for the cast film is time-consuming. When a plasticizer is used for insuring flexibility the product film, there occurs the problem of bleeding due to the incorporated plasticizer in addition to the above-mentioned disadvantages.
The method comprising melt-extruding polyvinyl alcohol in hydrous state is also disadvantageous in that a prolonged drying procedure is essential following the extrusion procedure. The use of a plasticizer for imparting flexibility to the extrudate causes the same bleeding problem due to the plasticizer in addition to the same other problems as mentioned above.
The method of melt-extruding much plasticized polyvinyl alcohol tends to cause bleeding-out of the plasticizer to make the surface of the product sticky or cause poor adhesion to other laminating sheets.
In contrast, vinyl alcohol polymers containing oxyalkylene units are as flexible as vinyl alcohol polymers per se and, therefore, do not require the addition of a plasticizer.
However, the oxyethylene group-containing polyvinyl alcohols described in U.S. Pat. Nos. 1,971,662, 2,844,570 and 2,990,398 referred to hereinbefore are disadvantageous in that it is difficult to control the length of the side chain oxyethylene group and the technique is unsatisfactory in productivity.
The oxyalkylene group-containing polyvinyl alcohols described in the specifications of U.S. Pat. Nos. 3,033,841 and 4,369,281 are not practically useful, for the graft-polymerization of alkylene glycol is low in reactivity.
The vinyl alcohol-poly(alkyleneoxy) acrylate copolymers described in U.S. Pat. Nos. 4,618,648 and 4,675,360, which are produced by copolymerizing vinyl acetate with poly(alkyleneoxy) acrylate and, then, hydrolyzing the copolymer, are disadvantageous in that the oxyalkylene group tends to leave off during the hydrolysis reaction so that the hydrolyzate is actually a mixture of the desired polymer and the liberated alkylene glycol or polyalkylene glycol.
Japanese Patent Application Kokai No. 59-155408 contains a description of the production process for an oxyalkylene group-containing vinyl alcohol polymer and of contemplated uses for the polymer but contains no specific disclosure of molding techniques, not to speak of melt-molding.
Japanese Patent Application Kokai No. 1-158016 suggests the melt-molding of an oxyalkylene group-containing vinyl alcohol polymer but when the proposed method is reduced to practice, smooth molding is hardly attained.
U.S. Pat. No. 4,824,904 and Japanese Patent Application Kokai No. 63-99209 disclose a saponified ethylenevinyl acetate copolymer including an oxyalkylene unit. However, the copolymer is insoluble in water.
Under the circumstances the present invention has been accomplished to provide a commercially useful method for melt-molding an oxyalkylene group-containing vinyl alcohol copolymer under substantially anhydrous conditions.